Biogen Idec Inc. (BIIB) increased the wholesale price of popular multiple-sclerosis drug Avonex by 6% last month, while Ireland's Elan Corp. (ELN, ELN.DB) gave a 7% boost to the price of MS therapy Tysabri, which it sells with Biogen.

The two blockbuster drugs make up the majority of Biogen's revenue and the price growth continues the trend of aggressive increases for drugs that treat the debilitating disease. The moves are the second increases for both drugs in the past eight months.

A Biogen spokeswoman declined to provide a reason for the move, saying that the company doesn't comment on its pricing strategy. An Elan spokesman said the Tysabri price increase "took into account multiple factors, including patient access, the reimbursement and the competitive pricing environments."

Biogen is in the midst of a restructuring that includes a focus on developing treatments for neurology and increasing sales of the existing MS treatments. Last year, the company completed the expansion of the Avonex sales force, along with other initiatives to reinvigorate U.S. sales of the drug, which was launched in 1996.

UnitedHealth Group Inc. (UNH) said in November that the prices for MS drugs have risen by 65% over the past three years. With the increases, the annual price of Avonex now stands at about $37,500 and Tysabri is $42,800, according to spokespeople from the companies.

Shares of Biogen recently rose 3.2% to $68.37, while American depositary shares of Elan fell 1.2% to $6.43.

Tysabri is Elan's biggest seller, although it is important to the future of both companies. Revenue from Avonex and Tysabri made up about 70% of Biogen's total of $4.4 billion in 2009.

Avonex is popular for initial treatment of the disease, while Tysabri is generally given to patients who don't respond to earlier therapy or have an aggressive form of MS.

In June, Elan increased Tysabri's price by 18.7%, marking the most aggressive increase since the drug's approval in 2004. Under their collaboration, Elan sets the price of Tysabri. The drug had global sales of $1.1 billion in 2009.

Tysabri's sales growth has been weaker than originally expected because of the drug's continued link to a rare brain infection called PML that led to its temporary market withdrawal in 2005.

For Avonex, price increases have aided sales growth as the number of new patients using the drug stagnated and competition increased across the market for MS treatments.

Lazard Capital Markets recently noted that Biogen last increased the price of Avonex by 4.5% in the third quarter and 5.5% last February, moves that followed three 9% price increases in 2009. A Biogen spokeswoman declined to confirm those price moves, citing policy.

The price increases in recent years have come with similar moves for other MS drugs including Teva Pharmaceuticals Industries Ltd.'s (TEVA, TEVA.TV) Copaxone, and Rebif, which is marketed by Pfizer Inc. (PFE) and Germany's Merck KGaA (MKGAY, MRK.XE). Late last year, Novartis AG (NVS, NOVN.VX) priced Gilenya, the first daily pill for MS, at $48,000 a year.

Biogen is in the midst of a restructuring that includes a focus on developing treatments for neurology and increasing sales of the existing MS treatments. Last year, the company completed the expansion of the Avonex sales force, along with other initiatives to reinvigorate U.S. sales of the drug, which was launched in 1996.

Last month, Biogen Chief Executive George Scangos downplayed the importance of price increases for the drugs. In an interview, he said there wasn't "a huge amount of room" for further increases.

"As we think about our planning, we think about market share and units and how do we compete effectively against the other drugs that are on the market, and less about what upside value we have in price increases," said Scangos.

-By Thomas Gryta, Dow Jones Newswires; 212-416-2169; thomas.gryta@dowjones.com